According to FDA guidelines, fish is considered sushi-grade when it has been frozen and stored at or lower than −20 degrees Celsius for 7 days, or other comparable temperatures and times. The reason for this being to ensure the death of parasites within the fish. Other fish are frozen only for convenience and are still intended to be cooked. All meats, not just fish, begin to spoil above certain temperatures, as do many other forms of produce and products. The quality of such products is also directly influenced by how low of a temperature they were frozen at. For example, it is widely accepted that the quality of fish, in particular sushi-grade fish, is maintained to a greater standard when frozen lower and quicker. Some even consider the quality of the fish to be enhanced by freezing in such ways. Water expands when freezing, thus causing degradation of texture and quality of products with high-water content. The slower and higher this freezing process occurs, the more degradation happens. It is also believed by some that the quality of certain meats begins to degrade above certain temperatures even if these temperatures are below the freezing point of water. It is easy to see that so-called “freezer-burn” is a common and unavoidable effect of storing items in temperature below yet close to the freezing point of water. These effects are decreased the lower the temperature is at which such products are placed. Thus, the ability to quickly freeze products, specifically meat, to extremely low temperatures and maintain such low temperatures is a desirable one, if not a required one.
In the transportation industry, current state-of-the-art methods for transporting frozen products are either unreliable or overly complex to manage. The typical transportation of fish in particular is extremely unreliable. Typical methods include placing ice or ice packs in, around, or on top of the fish to maintain its temperature. This isn't ideal because: 1. The freezing temperature for ice or typical ice packs is relatively high and can't absorb much heat energy, and 2. These methods cannot maintain extremely low temperatures for significant periods of time. While using ice to passively cool fish is extremely easy it is also extremely unreliable. While active methods are available, these methods are expensive. They include reefer trailers that cost tens of thousands of US dollars and are only capable of being used for the specific purpose of transporting by highway. The uses of such units are also limited by the standard compressor-based technology.
Current state-of-the-art technologies for transporting temperature sensitive products by air are bulky and extremely energy consumptive if they provide active thermal control, due to their compressor-based technology. Passive methods of cooling products during transportation through the air are unreliable in the same way that the typical transportation of fish is unreliable. Increased passive cooling capability occurs when dry ice is used in place of regular ice, but this method is complex and can be dangerous, and in general is impractical.
Certain freezing temperatures are required for many products' quality to be maintained within reasonable limits, specifically meat. This creates complications with the supply chain. These temperature requirements can greatly increase the complexity, energy consumption, and risk of quality loss. For cargo being transported by air specifically, it is preferable to decrease mass as much as possible, to not carry pressurized gases during flight, to not have large energy consumption requirements, and to not be transporting hazardous materials. This effectively rules out all methods of active freezing. Passive freezing can be utilized, but the most common methods—such as using ice, dry ice, gel packs, and insulation quilts—are imprecise and easily influenced by outside factors, thus putting at risk the quality of the product being transported. The need for a reliable and effective passive method of thermal control is demonstrated.
Cold chain methods often utilize different devices for thermal control throughout the cargo's lifetime. This creates complications. For example, a Bluefin Tuna is caught near Japan and blast-frozen on the fishing boat. This frozen tuna is transported out of these freezers into a storage freezer on the boat and then later to yet another storage freezer after the boat docks. The tuna is then moved to a shipping container where it is transported to a storage freezer in Los Angeles. It is then transported to restaurants, fish markets, or grocery stores where the consumer purchases it. Many steps are involved in which the tuna is transferred between freezers, allowing for greater chance of quality degradation to occur. This example is also assuming that the fish is always within a freezer except when being transported from one freezer to another. In practice, fish are often transported with only ice to maintain its low temperature, giving even more opportunity for quality degradation to occur. Thus, it is desirable to have a simpler method of maintaining low temperatures throughout the lifetime of the cold chain process of perishable goods.
Common methods for passive thermal control include packing perishables in water ice, dry ice, gel packs, or wrapping them in insulating quilts. These methods all require manual labor to be performed in order to position the perishables within or around the passive thermal control medium. Insulating quilts only provide a heat loss barrier. Water ice, dry ice, and gel packs maintain cold temperatures only temporarily through limited conductive heat transfer. Dry ice is the coldest of these materials, being at temperatures of around −80 degrees Celsius, and require special safety gear in order to avoid injury. Water ice will melt, becoming liquid that may damage perishables. Neither water ice nor dry ice is reusable, and while gel packs are reusable, they are only effective at temperatures above freezing. While such methods have been proven to work sufficiently, there is great potential for improvement in the passive thermal control of air transportation.
Another area of concern is the time it takes to transport large quantities of temperature sensitive products from one thermal control system to another. Having to transport products between freezers and transport units greatly increases the amount of heat energy that the products absorb due to the lack of thermal isolation from ambient temperatures which can exceed 100 degrees Fahrenheit. Any product whose temperature has already been steadily increasing throughout transportation will rise at an increasing rate not only during the time it itself is transported to the new thermal control system but the time it takes to transfer the rest of the products as well. A means of reducing the time spent transferring temperature sensitive products through high temperatures throughout the lifetime of a cold chain process is desirable.